To secure the centroiding accuracy of star image spots under the interference of sun straylight noise, we proposed a new centroiding method featuring the subtraction of background threshold and straylight slope noise in this paper. First, the gray noise of sun straylight was modeled as a slope within the window of a star image spot. For the new centroiding method, the error analysis equation of centroiding was deduced regarding the parameter estimation errors of the straylight slope model, especially the function relation concerning the window size. Then, the least-square parameter estimation formulas for the straylight slope model were given based on the window edge pixels. Finally, simulation tests were conducted through three centroiding methods, namely, the traditional gray weighted centroiding method (GWCM), the threshold subtracted GWCM, and the new method. The testing results show that the centroiding accuracy of the proposed method is two times and fifteen times higher than that of the two traditional methods. In conclusion, the method proposed in this paper is an effective centroiding method for star image spots under the interference of sun straylight noise and has a certain value in engineering applications.